by macrae11 » Thu Mar 26, 2009 11:51 am
Well I don't even really want to answer as there are certainly more qualified people than me on here, but I'll give you my small amount of knowledge. Others can add on and correct my mistakes afterwards.
First of all impedance is essentially resistance in an AC circuit. It's actually a combination of resistance, inductance and capacitance. One big difference between resistance in a DC circuit and impedance in an AC circuit, is the fact that impedance is frequency dependent.
Depending on the impedance of the drivers in the cabinet and how they're wired, series vs parallel, will give you the total impedance of the box. As you can see from amplifier specs the lower the impedance(resistance) there is the more power the amplifier will output. This is why it can get dangerous running more speakers than an amp is spec'd for. Some amps are capable of driving into a 2ohm load, but some will blow up pushing any serious amount of signal at 2 ohms. The good news if you're dealing with 8 or 16 ohm cabinets, there's not a lot to worry about unless you get stupid. Every time you double your speakers the impedance will be cut in half. 2 16ohm speakers become an 8 ohm load, 4 16ohm speakers become a 4 ohm load etc. With guitar cabinets this much simple math should keep you out of the woods. 16ohm 4x12, plug into the 16ohm output on the head. 2 4x12's, makes it 8 ohms, use the 8 ohm output.
Some cabinets also have impedance switching to go from 8-16ohms, so you can run more efficiently if using a half stack setup.
This is fairly basic, but hopefully I said things that make sense and don't confuse you more. I'll wait for Al or someone to come in and correct my mistakes.